It might actually be cheaper for the taxpayer to just
save the bank.
So, there are a few theories about why the financial crash happened:
Theory 1: The [banking system / market / bankers paid seven-figure salaries] actually had little to no understanding of what they were doing. Complicated derivative products and credit default swaps were mispriced because nobody really understands the maths and their risk models are all wrong.
This suggests, if we want to prevent the problem from recurring, the legislative intervention should be to either ban complex financial products or require much higher capital reserves for banks gambling on them.
Theory 2: The [banking system / market / bankers paid seven-figure salaries] had accurate models and understood the risks. They just decided they could go to the roulette table and put a trillion dollars on red, and the government would bail them out if the ball landed on black.
This suggests, if we want to prevent the problem from recurring, the legislative intervention should be to make bailouts radically less attractive for the people who benefit from such gambling. This could mean not providing bailouts, so banks that gamble and lose go bankrupt; or could involve some of the many other disincentives legislators have at their disposal.
If you follow this theory, and you adopt the policy that there will be no bailouts, you have to let the bank fail to prove to people that you really mean it.